Chaperone-mediated pathway of proteasome regulatory particle assembly

Abstract

The proteasome is a large proteolytic machine that degrades ubiquitin-tagged proteins. Substrates are recognized and unfolded by the regulatory particle (RP) and translocated into a central proteolytic chamber called the core particle (CP) where degradation takes place. The CP associates with the RP at either one or both ends. The RP can be further subdivided into the base and lid. Two studies from Finley and colleagues elucidate the pathway of RP assembly. They report the identification of three molecular chaperones that assist in the assembly of the RP. Assembly of the base proceeds through a complex consisting of five proteins, called BP1, which functions as an intermediate in the process. These studies show that RP assembly is a highly orchestrated process. The proteasome is a large proteolytic machine that degrades ubiquitin-tagged proteins. Substrates are recognized and unfolded by the regulatory particle (RP) and translocated into a central proteolytic chamber, called the core particle (CP), where degradation takes place. Here, and in a complimentary study, the pathway of RP assembly is elucidated, demonstrating that RP assembly is a highly orchestrated process. The proteasome is a protease that controls diverse processes in eukaryotic cells. Its regulatory particle (RP) initiates the degradation of ubiquitin–protein conjugates by unfolding the substrate and translocating it into the proteasome core particle (CP) to be degraded1. The RP has 19 subunits, and their pathway of assembly is not understood. Here we show that in the yeast Saccharomyces cerevisiae three proteins are found associated with RP but not with the RP–CP holoenzyme: Nas6, Rpn14 and Hsm3. Mutations in the corresponding genes confer proteasome loss-of-function phenotypes, despite their virtual absence from the holoenzyme. These effects result from deficient RP assembly. Thus, Nas6, Rpn14 and Hsm3 are RP chaperones. The RP contains six ATPases–the Rpt proteins–and each RP chaperone binds to the carboxy-terminal domain of a specific Rpt. We show in an accompanying study2 that RP assembly is templated through the Rpt C termini, apparently by their insertion into binding pockets in the CP. Thus, RP chaperones may regulate proteasome assembly by directly restricting the accessibility of Rpt C termini to the CP. In addition, competition between the RP chaperones and the CP for Rpt engagement may explain the release of RP chaperones as proteasomes mature.